File Systems Overview

A file system is a hierarchical structure (file tree) of files and directories. This type of structure resembles an inverted tree with the roots at the top and branches at the bottom. This file tree uses directories to organize data and programs into groups, allowing the management of several directories and files at one time.

Some tasks are performed more efficiently on a file system than on each directory within the file system. For example, you can back up, move, or secure an entire file system.

A file system resides on a single logical volume. The mkfs (make file system) command or the System Management Interface Tool (smit command) creates a file system on a logical volume. Every file and directory belongs to a file system within a logical volume.

To be accessible, a file system must be mounted onto a directory mount point. When multiple file systems are mounted, a directory structure is created that presents the image of a single file system. It is a hierarchical structure with a single root. This structure includes the base file systems and any file systems you create.

You can access both local and remote file systems using the mount command. This makes the file system available for read and write access from your system. Mounting or unmounting a file system usually requires system group membership. File systems can be mounted automatically, if they are defined in the /etc/filesystems file. You can unmount a local or remote file system with the umount command, unless a user or process is accessing that file system.

For information on the structure of the file system, see "Understanding the File Tree" .

File System Types

Multiple file system types are supported. These include the following:

Journaled File System

The native file system type is called the journaled file system (JFS). It supports the entire set of file system semantics. This file system uses database journaling techniques to maintain its structural consistency. This prevents damage to the file system when the system is halted abnormally.

Each journaled file system resides on a separate logical volume. The operating system mounts journaled file systems during initialization. This multiple file system configuration is useful for system management functions such as backup, restore, and repair, because it isolates a part of the file tree so that you can work on it.

Network File System

The network file system (NFS) is a distributed file system that allows users to access files and directories located on remote computers and use those files and directories as if they were local. For example, users can use operating system commands to create, remove, read, write, and set file attributes for remote files and directories.

CD-ROM File System

The CD-ROM file system (CDRFS) is a file system type that allows you to access the contents of a CD-ROM through the normal file system interfaces. It is a read-only local file system implementation under the AIX logical file system (LFS) layer supporting the following volume and file structure formats:

For all volume and file structure formats, the following restrictions apply:

• Single-volume volume set only
• Non-interleaved files only

The CDRFS is dependent upon the underlying CD-ROM device driver to provide transparency of the physical sector format (CD-ROM Mode 1 and CD-ROM XA Mode 2 Form 1), and the multisession format of the disks (mapping the volume descriptor set from the volume recognition area of the last session).

File System Commands

There are a number of commands designed to operate on file systems, regardless of type. The /etc/filesystems file controls the list of file systems that the following commands can manipulate:

Four commands operate on virtual file systems types. The /etc/vfs file contains the information on the file system types that the following commands manipulate:

File System Management Tasks

A file system is a complete directory structure, including a root directory and any subdirectories and files beneath it. File systems are confined to a single logical volume. Some of the most important system management tasks have to do with file systems, specifically:

• Allocating space for file systems on logical volumes
• Creating file systems
• Making file system space available to system users
• Monitoring file system space usage
• Backing up file systems to guard against data loss in the event of system failures
• Maintaining file systems in a consistent state.

Following is a list of system management commands that are used regularly for working with file systems: